The main purpose of the present study is to develop a simple method of measuring a parameter which is necessary for a radiation distribution model of fruit tree canopies (OLEAS), and which estimates extinction of photosynthetic active radiant flux density originating from derived diffused light (
PARFDdd) within the disorderly distributed leaves. Using shoots of several deciduous fruit trees, the disorderly distributed leaves with various
LAI were formed in a device made of two tubes. When direct sunlight went to one tube containing the leaves, the
PARFDdds were measured, and averaged (
I′). Here the
PARFDdds were extracted from a waveform of the output containing direct sunlight. The direct
PARFDs from another empty tube were measured, and averaged (
Io).
I′/
Io of each variety was distributed from 0.8% to 5.3%, when
LAI was about 1. In many varieties, the decrease of
I′/
Io became gentle when
LAI exceeded about 3. There was a low and negative correlation between mean value of
I′/
Io and
SPAD (mean relative concentration of chlorophyll) of the leaves. When
I′/
Io was approximated with
LAI,
LAI2 and
LAI3, the contribution ratio (
R2) of the equation of each variety was high generally, and was mostly over 0.94.
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